Static stability ($N^2$)

Resistance of a stratified fluid to a small vertical parcel displacement. For dry homogeneous air, $N^2=(g/\theta )d\theta /dz$ in terms of potential temperature; $N^2>0$ gives a restoring buoyancy force. Moisture and composition gradients modify the parcel comparison. Across an ideal sharp interface, a denser lower fluid is stable to an infinitesimal interchange.